Electronic ballasts for gas discharge lamps are often classified into two groups—preheat type and instant start type—according to how the lamps are ignited. In preheat type ballasts, the lamp filaments are initially preheated at a relatively high level (e.g., 7 volts peak) for a limited period of time (e.g., one second or less) before a moderately high voltage (e.g., 500 volts peak) is applied across the lamps in order to ignite the lamps. In instant start ballasts, the lamp filaments are not preheated, so a significantly higher starting voltage (e.g., 1000 volts peak) is required in order to ignite the lamps. It is generally acknowledged that instant start type operation offers certain advantages, such as the ability to ignite the lamps at a lower ambient temperature and greater energy efficiency (i.e., greater light output per watt) due to no expenditure of power on filament heating during normal operation of the lamps. On the other hand, preheat type operation usually results in considerably greater lamp life than instant start type operation.
Within the group of ballasts that are classified as preheat type ballasts, there are two main categories—rapid start ballasts and program start ballasts. Program start ballasts are generally preferred over rapid start ballasts, mainly due to the fact that the amount of energy that is expended upon heating the lamp filaments during normal operation is generally significantly reduced in those types of ballasts.
Preheat type ballasts typically include one or more resonant output circuits. The one or more resonant output circuits serve to provide a number of functions, such as preheating of the lamp filaments, providing a high voltage for igniting the lamp(s), and supplying a magnitude-limited current for powering the lamp(s) during steady-state operation. In order to preserve and optimize the useful operating lives of the lamps, it is important for the ballast to provide the lamps with appropriate levels of filament preheating and with an appropriately high ignition voltage for igniting the lamps.
Program start ballasts typically employ a circuit that includes a high frequency inverter and a resonant output circuit. As is known to those skilled in the art, the effective resonant frequency of a resonant circuit is dependent upon certain parameters, including the inductance of the resonant inductor and the capacitance of the resonant capacitor. In practice, those parameters are subject to component tolerances, and may vary by a considerable amount. Additionally, the effective resonant frequency of a resonant circuit is also influenced by the lead lengths and/or the nature of the electrical wiring that connects the ballast to the lamp(s); the electrical wiring introduces parasitic capacitance (also referred to as “stray capacitance”) which effectively alters the effective resonant frequency of the resonant circuit(s), and which therefore affect the magnitudes of the preheating and ignition voltages provided by the ballast to the lamp(s). Such parameter variation makes it difficult and/or impractical to pre-specify (i.e., on a priori basis) an operating frequency of the inverter so as to ensure that suitable preheating and ignition voltages are provided to the lamp(s).
The aforementioned difficulties arising from parameter variation are even more problematic when the ballast includes multiple resonant circuits and/or when the wiring between the ballast output connections and the lamps has a considerable length; in the latter case, the resulting parasitic capacitance becomes a very significant factor. Accordingly, for a given predefined inverter operating frequency, the magnitudes of the filament preheating and ignition voltages that are provided by a resonant output circuit may vary considerably, and may, in some instances, prove to be either insufficient or at least considerably less than ideal, for preheating and igniting the lamp in a desired manner.
Thus, a need exists for a program start type ballast that is capable of compensating for the parameter variations that affect a resonant output circuit, so as to ensure that the ballast provides both an appropriate level of preheating for the lamp filaments, as well as a sufficiently high ignition voltage for igniting the lamp(s). A ballast with such capabilities, and that is capable of being realized in a convenient and cost-effective manner, would represent a considerable advance over the prior art.